Detection of catastrophic failure of dielectric, improper connection, and temperature of a printed circuit assembly via one wire

ABSTRACT

A first sensor for the detection of dielectric failure (by burning) within a multilayer printed circuit assembly comprises an isolated conductive layer. This first sensor is connected by a first diode to a single wire which also connects a second, temperature, sensor via a second diode (system ground is a return). A multiplicity, nominally 16, of such single wire connected sensor pairs are selectable in accordance with an externally (microprocessor) furnished address. During a first time period, an externally (microprocessor) selected interrogation of temperature causes a first, positive, voltage bias to be applied to the selected sensor pair resulting in a current linear with temperature (over the range of 0° C. to 100° C.) in the second sensor. This current is transformed to voltage, offset by 273° Kelvin, amplified, and converted to a digital value for issuance to an external (microprocessor) requestor. During a second time period, an externally (microprocessor) selected interrogation of dielectric failure causes a second, negative, voltage bias to be applied to the selected sensor pair. Any current sensed--which represents failure shorts between the isolated conductive layer and any other voltage, ground, or signal within the multilayer printed circuit assembly--is transformed to voltage, amplified, digitalized and issued externally. If neither temperature nor dielectric leakage currents can be properly sensed, and especially for plural addressable sensor pairs upon the same printed circuit assembly, then the assembly is deemed to be unconnected or improperly pluggably connected. Temperature (over temperature), dielectric failure, and failed connection are cyclically continuously monitored for a multiplicity of sensor pairs upon a plurality of printed circuit assemblies with an individual sensor test, or interrogation, time of 50 milliseconds.

BACKGROUND OF THE INVENTION

The present invention relates generally to fault condition monitoring ofprinted circuit assemblies, such as those utilized within a digitalcomputer, and specifically to the detection of the catastrophic failureof the dielectric in a multilayer printed circuit assembly, to thedetection of the improper pluggable connection or non-connection of sucha printed circuit assembly, and to the detection of temperature (overtemperature) occurring on such printed circuit assembly. All suchdetections occur via a single signal wire, using system ground as thereturn.

Modern digital computers often use an array of multilayer printedcircuit boards or assemblies to hold the digital logic components,interconnect them, and provide power to them. The amount of power usedin such arrays of printed circuit assemblies is very high, on the orderof tens of kilowatts. If a breakdown in the printed circuit dielectricbetween voltage and ground occurs, sufficient power is available tocause burning (carbonization) and potential fire on the failed printedcircuit assembly. Such a catastrophic failure and fire may damage otherclosely spaced printed circuit assemblies.

The prior art burn detection method of smoke, or combustion product,detectors suffer from unreliability and a slow response time relative tothe speed of the burn. Also previously utilized, overcurrent detectionin power supplies is now insufficient to protect an individual printedcircuit assembly. This is because the power supply may be outputtingsufficient current to allow the catastrophic burn of an individualmultilayer printed circuit assembly without such sufficient excess, orover, current as may be detected and utilized to protect such assembly.Finally, it would be possible to fuse each individual printed circuitassembly but such fusing is often inefficient or impractical. Fuses havea voltage drop and interfere with regulation of the d.c. voltage source.Fuses are physically large for the currents involved, approximately 100amperes, and do not fit on a printed circuit board. Finally, if anindividual printed circuit card assembly is fused at full circuitcurrent, a burn may still occur and not blow the fuse.

As second and third aspects of the present invention the detection of animproper connection of a pluggable printed circuit assembly, and thetemperature condition, including over temperature, occurring upon suchprinted circuit assembly will be detected upon a single signal wire,such single wire as is additionally utilized for the detection ofcatastrophic failure of the dielectric of the multilayer printed circuitassembly. Improper connection of a pluggable printed circuit assemblymeans that the connector(s) of such assembly, such as a linear plug, is(are) not physically correctly inserted to allow contacting the properpins, or printed circuit lands, upon the printed circuit assembly. Tothe maximum extent possible, such improper plug connection of a printedcircuit assembly is normally precluded by a physical design of theconnectors so that improper mating is precluded. Insofar as electricalmethods detect and verify the proper connection of a pluggable assembly,a check of the continuity through a first end pin of the connector,across a proscribed path upon the printed circuit assembly, and out anopposite end pin of the connector might be performed in order todemonstrate that the printed circuit assembly was properly plugged atleast at each end of the connector. If one connected pin, possibly anend pin, is ground, then the proper occurrence of ground, as routed bythe printed circuit assembly, upon another pin, possibly the other endpin, may be observed as an indication of correct connection. Finally,some signal(s) developable upon the printed circuit assembly only in theevent of proper connection may be observed as an index (indices) of suchproper connection. Should such electrical checks for proper connectionbe performed at all, they would not usually be perceived as beingassociated with either the sensing of temperature (detection of overtemperature) nor the detection of the catastrophic failure of adielectric in a multilayer printed circuit assembly.

Finally, the sensing of temperature (detection of over temperature)occurring upon a multilayer printed circuit assembly by theinterrogation of a sensor, such as a thermocouple or thermistor, locatedupon such multilayer printed circuit assembly is old in the art.Normally, one port of a two port sensor is connected to either voltageor ground and only the signal resultant at the other port needs berouted from a pluggable multilayer printed circuit assembly for theexternal sensing of temperature occurring upon such assembly. When sucha single wire connection is utilized for the sensing of temperature upona printed circuit assembly, it is not normally associated with anyadditional purpose(s).

SUMMARY OF THE INVENTION

The present invention allows the detection of the catastrophic failureof the dielectric within a multilayer printed circuit assembly. Suchdetection requires one wire, or connector contact, with ground as thereturn path. Upon this same wire the improper connection, ornon-connection, of the printed circuit assembly as pluggably connected(non-connected) to a multi-position connector may additionally besensed. Finally, a sensor detecting temperature and over temperatureupon the multilayer printed circuit assembly may additionally be sensedthrough the same wire interconnect.

The preferred embodiment detection of the catastrophic failure of thedielectric in a multilayer printed circuit assembly involves the threeelements of (1) an isolated conductive plane connected via a (2) diodeto a (3) voltage biasing (in a first direction) and sensing means. Theisolated conductive plane, normally copper, is employed throughout thearea of each logic board, side panel, and back panel multilayer printedcircuit assembly which is subject to test for failure of the dielectric.This isolated conductive plane, or sensor layer, will normally "float"with more than one megohm of resistance to any other voltage, ground, orsignal in the protected logic boards. This separate layer may bemonitored directly, without voltage biasing or diode isolation means,for showing low resistance to any other part of the multilayer printedcircuit assembly circuitry, thereby indicating a failure breakdown ofthe dielectric, responsively to which dielectric breakdown power isnormally turned off to preclude further charring and potential fire. If,however, the detection of low resistance shorts between this isolatedsensor reference layer and any other voltage, ground or signal withinthe multilayer printed circuit assembly is desired to be combined, forefficiency of pin and wire utilization, with additional functions (ofadditional sensors) such as proper connection and temperature, then thisisolated reference sensor layer is normally connected to a voltage biasand sense means through an isolation diode. Such voltage biasing meanswill forward bias the connective diode in order to sense for lowresistance shorts to the reference sensor layer. The diode stops thereference sensor layer's normal leakage from affecting any otherutilization of the same sense wire when the isolation diode isback-biased.

When such diode isolation of a reference sensor layer for the detectionof dielectric failure in a multilayer printed circuit assembly isemployed, then a diode isolated temperature sensor may be employed uponthe same wire connection. The voltage biasing and sensing meansconnected to this wire will at one time provide a first bias voltage forforward biasing the diode connecting to the sensor reference layer inorder to allow first sensing for low resistance failure shorts to suchlayer. The voltage biasing and sensing means will at a second timeprovide a second, reverse, voltage bias which will forward bias thediode connecting to the temperature sensor and permit the second sensingof the output of such temperature sensor. During the period of firstvoltage biasing (for sensing of current to the sensor reference layerfor the detection of the catastrophic failure of the dielectric), thediode isolating the temperature sensor will be back-biased and thuslythe temperature sensor will draw no current and will not interfere withsuch first sensing. During the second period voltage biasing (forchecking of the temperature sensor), the diode isolating the referencesensor layer will be back-biased, and thusly the reference sensor layerwill draw no current and will not interfere with such second sensing.Thusly, by utilizing at a first time a first voltage region for thesensing of the catastrophic failure of a dielectric in a multilayerprinted circuit assembly, and by utilizing at a second time a secondvoltage region for the sensing of the temperature upon such printedcircuit assembly, a common wire connection may be utilized for bothpurposes.

The same voltage biasing and sensing means which in a first voltage biasregion senses that low resistance which indicates dielectric failure,and which in a second voltage bias region senses that resistance whichindicates temperature, can, through such sensing, support thedetermination that the subject printed circuit assembly is improperlyconnected, or not connected at all, if neither current can be sensed asanything abnormal. Thusly, a single voltage biasing and sensing meanswhich first voltage biases to a first voltage region in order to sensethe resistance of a sensor reference layer, and second voltage biases toa second voltage region in order to sense the resistance of atemperature sensor, provides respective dielectric breakdown andtemperature information, which if considered jointly, is also indicativeof the correct printed circuit assembly pluggable connection. By such acombinatorial utilization of obtained observations, it is more effective(as well as more efficient in wires, connector pins, and voltage biasingand sensor circuits employed) to exercise the functions of detection ofthe catastrophic failure of dielectric, improper connection, and overtemperature of a printed circuit assembly jointly collectively via onewire than to perform such sensings separately.

Correspondingly, it is a first object of the present invention that amultilayer printed circuit assembly will be checked for the occurrenceof the failure of the dielectric and/or a catastrophic burn whereinshorts develop between the printed circuit lands which carry voltagesand those lands as variously carry signals and ground. It is a secondobject of the present invention that the monitoring of temperature upona printed circuit assembly will be accomplished in a time-multiplexedfashion across the same single wire which is elsetimes utilized for thedetection of a dielectric failure upon such printed circuit assembly. Itis a third objective of the present invention that the nonconnection, orimproper pluggable connection, of a printed circuit assembly may bedetermined by the responses received upon that same single wire which isutilized for either the detection of dielectric failure and/or themonitoring of temperature upon such printed circuit assembly.

It is a fourth objective of the present invention that the sensing ofdielectric breakdown and/or temperature and/or improper connection of aprinted circuit assembly (whether jointly sensed in a time-multiplexedmanner via a single wire or not) as detected by a multiplicity ofsensors upon a plurality of printed circuit assemblies should beaccomplished in a selectable addressable fashion. Such selectableaddressing and interrogation of each of a multiplicity of sensors,normally continuously accomplished in cyclic rotation for the detectionof incipient fault conditions, is enabled responsively to an addresssupplied from an external microprocessor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, consisting of FIG. 1a and FIG. 1b, shows the apparatus of thepresent invention consisting of a diagrammatically illustrateddielectric failure sensor and a schematically represented temperaturesensor/transducer which are jointly commonly connected to aschematically represented electronic circuit such as allows thedetection of the catastrophic failure of the dielectric, the improperconnection, and/or the temperature (over temperature) of the printedcircuit assembly.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the past, air cooled computers sensed printed circuit assembly highassembly temperature and high air temperature faults via the use ofmechanical thermostats. These mechanical thermostatic switches weremounted in the main airstream as it left the modules being monitored, orupon the modules themselves. Because of the mixing effect of theairstream above the printed circuit assemblies, a few distributedsensors could provide adequate protection. Previously utilizedmechanical thermostats are very slow acting in air with a time constantof several minutes and a nominal accuracy of ±3° C. More exactingmonitoring and protection of individual printed circuit assemblies wasnot previously considered necessary or practical. The more exactingtemperature monitoring and high over temperature fault detection schemeof the present invention utilizes an electronic temperature transducer,such as may be commercially purchased as type AD 590J available fromAnalog Devices, Inc., or such as is taught within U.S. patentapplication Ser. No. 395,516 entitled COOLING SYSTEM ASSEMBLY AND METHODto T. P. Currie and T. B. Zbinden. The preferred embodiment temperaturetransducer is implemented in die form, and is mounted in a standard sizesingle scale integrated circuit package upon the printed circuitassembly. The physical dimension of the die cavity of the temperaturesensor/transducer taught in U.S. patent application Ser. No. 395,516 isapproximately 0.34 centimeters by 0.34 centimeters (0.135 inches by0.135 inches). The temperature sensor/transducer is a linear outputintegrated circuit current type device which sinks one microampere perdegree Kelvin. The use of current type devices eliminates the effect ofground loss. A single signal contact per printed circuit assembly isneeded to transmit, as a current signal, the temperature of the printedcircuit assembly to further circuitry of the present invention, called ascan circuit, which may be remotely situated upon a separate card. Thetemperature sensor/transducer of the preferred embodiment of theinvention, such as is intended to be utilized in a liquid coolant cooledcomputer, is calibrated and sensed for the determination of temperatureover the range of approximately 0° C. to 100° C., although theapplication of the principles of the present invention to othertemperature sense ranges is possible.

Also in the past, fire detection in multilayer printed circuitassemblies had been done with smoke detectors which were unreliable, orwith the detection of over current within power supplies. Modern digitalcomputers often utilize an array of multilayer printed circuitassemblies to hold the circuit components, interconnect them, andprovide power to them at high densities and close conductor spacings.The mount of power utilized in such new arrays of multilayer printedcircuit assemblies is very high, and it has been found that if abreakdown in the dielectric between a voltage and ground occurs,sufficient power is available to cause the catastrophic dielectricbreakdown and a potential fire upon the failing multilayer printedcircuit assembly. Individual circuit fusing of the multilayer printedcircuit assemblies is not feasible, so the apparatus of the presentinvention provides a means of detecting this dielectric failure andensuing burn before it can cause severe damage to other circuitassemblies, property and personnel. The method of the present inventionutilizes a separate, isolated, conductive layer, such as of copper,inside the multilayer structure of the multilayer printed circuitassembly. When this normally isolated separate burn sensor layer shows alow resistance to any other parts of the multilayer printed circuitassembly, indicating a breakdown of the dielectric, a failure has beenfound and the power can be turned off prior to the development of afurther, catastrophic, burn.

Two separable and severable systems concepts are applied in theutilization of the circuit of the present invention for theinterrogation and interpretation of the temperature sensor/transducerand the burn detection sensor layer (as may each be located upon asingle multilayer printed circuit assembly). These separable and severalsystem concepts are, however, most efficacious for the further detectionof the improper pluggable connection of such a printed circuit cardassembly when such systems concepts are both implemented. The firstsystem-type concept of the circuit of the present invention is that boththe temperature sensor/transducer (such as indicates the temperatureoccurring upon a printed circuit assembly) and the isolated burn sensorlayer (such as indicates the occurrence of a dielectric breakdown withina multilayer printed circuit assembly) may be interrogated in atime-multiplexed fashion via a single wire. This is accomplished by acircuit, the previously identified scan circuit, which provides avoltage bias (nominally positive) during a first time period via thesingle wire connection (with ground as the return path) for the sensingof the temperature sensor/transducer upon the printed circuit assembly,and which first derives the temperature therefrom such sensing. During asecond time period a complementary voltage bias (nominally negative)will be provided by the scan card circuit via the same single wire tothe multilayer printed circuit assembly for the purpose of detecting lowresistance shorts between the isolated burn sensor layer and any othervoltage, ground, or signal within the multilayer printed circuitassembly. The temperature sensor/transducer and the isolated burn sensorlayer are each isolated by an associated diod. During the application ofthe first voltage bias (nominally positive) the diode isolating the burnsensor layer is reverse biased whereas the diode isolating thetemperature sensor/transducer sensor element is forward biased. Therebythe current resultant from the temperature sensor/transducer sensorelement will be sensed while any current leakage, whether normal orabnormal, in the isolated burn sensor layer will be blocked by thereverse biased diode from affecting such temperature sensing.Conversely, when the second voltage bias (nominally negative) isapplied, the diode isolating the burn sensor layer is forward biasedwhereas the diode isolating the temperature sensor/transducer sensorelement is reverse biased. In such a second bias condition, no currentwill flow in the temperature sensor/transducer sensing element and theonly current sensed will be that occurring between the isolated burnsensor layer and other voltages, grounds, or signals within themultilayer printed circuit assembly. Thus, the first systems conceptapplied is that the detection of the catastrophic failure of thedielectric and the temperature (over temperature) of a multilayerprinted circuit assembly may be accomplished in a time-multiplexedmanner via a single wire.

It is the separable and severable second systems concepts in the circuitof the present invention that the sensing of dielectrick breakdown andtemperature (whether jointly sensed in a time-multiplexed manner via asingle wire in accordance with the first systems concept or not) upon aplurality of printed circuit assemblies should be accomplished in aselectable addressable rotation. Such rotational addressable sensing ofa multiplicity of dielectric failure and temperature type sensors isaccomplished by the scan circuit of the present invention responsivelyto addresses supplied from an external microprocessor. The scan circuitof the present invention, so-named because it allows the multiplicity ofsensors upon the plurality of printed circuit assemblies to beaddressably interrogated or scanned, looks like a memory to suchmicroprocessor wherein each sensor is an address.

Such a second systems concept, especially in conjunction with the firstsystems concept of time-multiplexed sensing of dielectric failure andtemperature, allows the detection of improperly inserted and connectedprinted circuit assemblies. If for all addresses sent to the scancircuit by the microprocessor no response is received, then the scancircuit itself has failed. If the first systems concept is employed,thereby sensing dielectric failure and temperature via a single wire ata single address, and a response to both such sensings equivalent to animproper reading is detected then the strong implication is that theselected printed circuit assembly is improperly electrically connected.This conclusion is especially strong if two such addresses (as areconnected by two wires) communicate to two dielectric failure andtemperature sensor circuits upon the same printed circuit assembly suchas can transpire when the second systems concept is employed. It isexceedingly unlikely that an identical sensing, equivalent to animproper reading should be sensed at the two addresses corresponding tothe two such sensors. The employment of both systems concepts within thepreferred embodiment of the present invention permits of the unambiguouselectrical determination that a printed circuit assembly is absent orimproperly connected. Such a determination is normally used by a powercontrol system (not part of the present invention) to remove power fromthat pluggable location at which a printed circuit assembly is eitherabsent or improperly connected.

The diagrammatic and schematic representation of the present inventionis shown within FIG. 1. The physical structure of the multilayer printedcircuit assembly subjectable to dielectric failure testing isdiagrammatically represented as TOP PAD LAYER, ISOLATED BURN SENSORLAYER and PRINTED CIRCUIT BOARD LESS TOP PAD LAYER. Such multilayerprinted circuit assembly need not be structured identically in the threeparts as illustrated, the sole requirement being that an electricallyconductive layer isolated from all other signals, voltages, and groundupon the multilayer printed circuit assembly be established. The diodeconnection to the isolated burn sensor layer via diode CR1 type 1N4150needs be accomplished only if the temperature sensor/transducer assemblyS1 is jointly connected via a single wire (connected through plug jackPN to the analog multiplexor MUX which commences the scan circuit). Thetemperature sensor/transducer assembly S1, which is old in the art, ispart number AD590J available from Analog Devices, Inc. This temperaturesensor/transducer S1 sinks an essentially linear one microampere perdegree Kelvin to ground GND when biased within the range of +4.5 to +30volts d.c. The temperature sensor/transducer assembly S1 contains anintegral diode as illustrated, and is mounted as a die contained withina normal integrated circuit package upon the printed circuit assembly.

The remaining circuitry as is shown in FIG. 1 accomplishes the dualsystem's purposes of allowing the time-multiplexed sensing of dielectricfailure and temperature upon a single wire, and the addressableselection amongst a multiplicity of such wires. The analog multiplexorMUX, the latches L2, and the decoder D1 are primarily involved with thesecond system's purpose of allowing selection amongst a multiplicity ofsensors. These circuit elements could be eliminated from the scancircuit of the present invention, with a single analog sensor signal asappears at plug jack PN being directly passed for amplification inbuffer amplifier AMP1, offset amplification in offset differentialamplifier AMP2, and conversion to a digital signal in analog to digitalconverter CON.

That portion of the scan circuit as shown in FIG. 1 concerned with theimplementation of the first systems purpose of time-multiplexed sensingof both dielectric failure and temperature via a single wire resides inthe area of latch L1 through resistor R3, including transistors Q1 andQ2. This portion of the scan circuit will, under the selection controlof a digital signal received from an external microprocessor MP and asgated by timing T1, apply at a first time a first positive voltage biasand apply at a second time a second negative voltage bias to thedielectric failure sensors and the temperature sensor/transducer S1 asare jointly connected through plug jack PN. The preferred embodimentimplementation of the invention as shown in FIG. 1 can be altered by theelimination of either that portion of the apparatus as serves the firstsystem's purpose or the second system's purpose. For example, ifaddressable selection, such as is ultimately enabled under control ofexternal microprocessor MP, is deleted then the remaining circuit shownis still operative for the combinatorial sensing of dielectric failureand temperature via a single wire. Alternatively, if temperaturesensor/transducer S1 (which is old in the art) were deleted (leavingonly the isolated burn sensor layer for the detection of dielectricfailure, which needs no longer be isolated by diode CR1) and the biasingcircuitry consisting of latch L1 through resistor R3 was notimplemented, then the remaining circuit of FIG. 1 would still show theability to selectively addressably sense a multiplicity of suchdielectric failure sensors as may be located upon a plurality ofmultilayer printed circuit assemblies. Thusly, it may be seen that thepreferred embodiment of the present invention as shown in FIG. 1 hascertain aspects which are physical and structural, notably the isolatedburn sensor layers for the detection of dielectric failure, andadditional aspects, notably the duality of sensing dielectric failureand temperature in a time-multiplexed manner plus the selectableaddressable sensing of a multiplicity of such conditions as may occurupon a plurality of printed circuit assemblies, which are electrical,and oriented toward the system application of such sensing, in nature.In other words, the preferred embodiment of the present invention asshown in FIG. 1 shows which basically physical fault phenomena todetect--dielectric failure, over temperature and improperconnection--and how to electrically detect them and how to efficientlyeffectively combinatorially detect these faults as may occur upon anarray of a multiplicity of printed circuit card assemblies.

Commencing with the detailed electrical explanation of the circuit ofthe preferred embodiment of the present invention for the detection ofthe catastrophic failure of the dielectric, the temperature, and theimproper connection of a printed circuit assembly, which circuit isshown in FIG. 1, all electrical components shown are standardcommercially available parts. Resistors are labeled with their value inohms, and capacitors with values of one or greater are expressed inpicofarads whereas values less than one are in microfarads. Thecircuitry between the plug jacks connecting to the sensors, such as plugjacks P1 through P16 shown, and the plug jacks connecting to themicroprocessor MP, shown as J1 through JN, is called a scan circuitwhich is normally itself implemented upon a pluggable printed circuitassembly. The current signal from the dielectric failure sensor (duringthe application of negative bias) and the temperature sensor/transducerS1 (during the application of positive bias) are received at analogmultiplexor MUX industry standard part number 506. The connection of onesuch sensor pair via a single wire is shown through plug jack PN, one ofsixteen plug jacks P1 through P16 connecting to analog multiplexor MUX.Each signal line is protected against over voltage transients due toelectromagnetic pulse or electrostatic discharge by two reverse biaseddiodes: illustrated diode CR2 connecting the signal line of plug jack PNto voltage source +15 V and diode CR3 connecting the same signal line tovoltage source -15 V, such diodes as are typical of all signal lines.The analog multiplexor MUX is selected to connect one of sixteenpossible signal lines, such as that signal line connecting upon plugjack PN, to input pin 2 of buffer amplifier AMP1 under the selectioncontrol of signals applied from decoder D1. The selection signals fromdecoder D1 applied to analog multiplexor MUX, and potentially to furtherlike analog multiplexors as support further groups of sensors, arederived responsively to decode of the address contained within latchesL2. Such an address contained within latches L2, interpretable for theselection of an individual sensor pair, is derived from digital addresssignals sent from microprocessor MP via bidirectional signal linesconnecting to plug jacks J1 through JN-1, which signals are used to setvarious ones of latches L2. The time upon which such address signalsfrom microprocessor MP are lodged in latches L2 is controlled by timingT1, such timing T1 as is not a standard part but which is rather thelogical circuit implementation of the communications protocoltranspiring on the bidirectional digital communication bus between thescan circuit and microprocessor MP, in accordance with whatsoeverdigital communications protocol is employed upon this interface. Theconstruction of such a communication bus is routine in the art. Alsodeveloped in timing T1 is a gating signal to latch L1 allowing thereceipt of a signal from microprocessor MP via the bidirectionalcommunication line through plug jack JN, and a momentary pulse signal tothe analog to digital converter CON at such time as the scan circuitshould form a final digitalized output to be sent via the linesconnecting through plug jacks J1 through JN to microprocessor MP. Fordetermination of which and when these various enablements should beperformed, the timing circuit T1 receives the signals generated at themicroprocessor MP which appear upon signal lines connecting through plugjacks J1 through JN-1. In accordance with the interface protocol for thedigital communication bus between the scan circuit and themicroprocessor MP, the timing T1 may also receive lines such as "dataready to be received" by microprocessor MP, and may issue controlsignals such as "data ready to be sent" to microprocessor MP. The scancircuit of the present invention as shown in FIG. 1 utilizes a 50millisecond delay to allow the setting and conversion of the analogsignals. In other words, the time upon which timing T1 will cause boththe address of the selected sensor pair to become lodged in latches L2,and the selection for sense of one such sensor at said address via thegating of the signal setting (or clearing) latch L1, until such time asthe conversion enablement pulse from timing T1 to analog to digitalconverter CON will become logically false and thus a steady digitalizedvalue will be offered by analog to digital converter CON tomicroprocessor MP, will be 50 milliseconds.

Normal employment of the scan circuit and connected sensor pairs by themicroprocessor MP will be to continuously cyclically address amongst themultiplicity of connected sensors in order to inspect for the dielectricfailure or over temperature fault conditions during the entireoperational lifetime during which the connected multilayer printedcircuit assemblies are utilized. A large multiplicity of dielectricfailure and/or temperature sensors arrayed upon a large plurality ofprined circuit assemblies may thus be cyclically interrogated within acycle period which is suitably short in time (50 milliseconds persensor) for the adequate protection from catastrophic dielectric failureand over temperature of each of such printed circuit assemblies. Theprogram which runs in microprocessor MP can selectively (rotationally,if desired) interrogate ones of the dielectric failure and temperaturesensors (receiving digital results through the scan circuit) and make adetermination based on results received as to non-connection (improperconnection) or dielectric failure and/or over temperature of a printedcircuit assembly. The microprocessor MP will normally be connected tocontrol the power source and coolant source to the printed circuitassemblies (not shown). Normally, a printed circuit assembly will not bepowered on unless it can be sensed as properly connected, and would bepowered down for the duration of a dielectric failure or overtemperature condition (through the control of a program operating inmicroprocessor MP operating through power control means not shown).Since the circuit of the present invention provides for addressing anabundance of interrogatable sensors, it is normally desirable to createone addressable location to which no sensors are connected, oneaddressable location connecting to a current source simulatingdielectric breakdown, and some number of current sources as simulate theoccurrence of various high temperatures. The program operating withinmicroprocessor MP may thusly observe the response of the scan circuit asshown in FIG. 1 to these various simulated fault conditions. A typicalsystems application of the circuits and methods of the present invention(possessing both sensing and addressing aspects) may thusly be seen tooffer a high degree of system verification and safety, the potentialexisting to validate correct operability within all functional areas.Multiple sensors reflective of separate addresses may be employed foreach individual protected printed circuit assembly. The scan circuititself (normally located as a pluggable printed circuit assembly) may bereplicated in duplicate for parallel interconnection to parallel sets ofsensors. When such systems level redundancy in the function of thepresent circuit is combined with the fast time performance of thepresent circuit, a high degree of confidence in the system physicalstatus as would besuit the protection from fault of very expensiveand/or extensive printed circuit assemblies may be obtained.

Continuing in FIG. 1, the scan circuit, so-named because it allows thedielectric failure sensors and/or the temperature sensors to beaddressably referenced or scanned, serves as the interface between amultiplicity of such current generating analog sensors and a digitalinterface to the microprocessor MP. A typical analog current input,connecting via a single wire to both the dielectric failure sensorisolated by diode CR1 type 1N4150 and the temperature sensor/transducerS1 type AD 590J available from Analog Devices, Inc. (which contains adiode) is shown as connector plug jack PN. This signal line, one ofsixteen possible into analog multiplexor MUX digital integrated circuittype CMOS 506 is isolated against electromagnetic pulse andelectrostatic discharge by diodes CR2 and CR3 which are power diodestype BV-125 V IN458. This addressably selected analog signal line isswitchably connected in analog multiplexor MUX to pin 2 of thedifferential input buffer amplifier AMP1, which is an integrated circuitoperational amplifier type TTL 741. The voltage bias at pin 2 of thebuffer amplifier AMP1, a positive voltage bias for the sensing oftemperature and a negative voltage bias for the sensing of dielectricfailure, both voltage biases as are fed through analog multiplexor MUXback to the selected sensors upon the printed circuit assembly, isdeveloped in the bias circuitry proceeding from latch L1 to resistor R3.The latch L1 is a digital integrated circuit flip-flop dual D type TTLS74LS74, and is set or cleared by the presence or absence of a digitalsignal from microprocessor MP received through plug jack JN as gated bya signal from timing T1. A voltage bias to the base of transistor Q2,silicon NPN type VCBO 25B 300 MW Beta 30, is obtained from resistor R8type fixed film 0.25 W 2% 10K, diode CR6 type PWR IF 200 MA BV- 30 VIN914, and resistor R9 type fixed film 0.25 W 2% 10K. When latch L1 iscleared impressing a logical Low, or 0 volt d.c., upon the cathode ofdiode CR7 type PWR IF 200 MA BV IN914 then 0 volts d.c. appears upon thebase of transistor Q2 and such transistor is turned off, ornon-conducting. In such an eventuality, +15 volts d.c. appears throughresistor R6, fixed film type 0.25 W 2% 10K, to the base of transistor Q1type silicon NPN VCBO 25 V 300 MW BETA 30 and makes it non-conductingalso. Conversely, if latch L1 is set, the logical High, approximately +3volt d.c., signal output applied to the cathode of diode CR7 type PWR IF200 MA BV- 30 V IN914 will enable a voltage to be developed at the baseof transistor Q2, turning such transistor on into saturation.Resultantly to the conduction of transistor Q2, the voltage dividercomposed of resistors R6 and R7, types fixed film 0.25 W 2% 10K, willdevelop a voltage at the base of transistor Q1 and turn it on intosaturation. Transistor Q1 may thusly be turned on or off according tothe setting of latch L1.

When transistor Q1 is turned off, the scan circuit operates in the burnsensing mode for the detection of dielectric failure. The voltagedivider between -15 volt d.c. and ground GND, consisting of resistancesR10 and R11, both types fixed film 0.25 W 2% 4.3K, serves to emplaceapproximately -7.5 volts d.c. on pin 3 of buffer amplifier AMP1. Theoperation of buffer amplifier AMP1 will serve, through the feedback loopconsisting of diode CR4 type PWR IF 200 MA BV- 30 V DN966, and resistorR2 type fixed film 0.25 W 2% 2.2K to force pin 2 to the same voltage asis applied to pin 3, mainly -7.5 volts d.c. There will be no voltageacross zener diode CR5, semiconductor device type ZENER 6.20 V nominal5% 400 MW. The -7.5 volts d.c. voltage appearing at pin 2 of bufferamplifier AMP1 is passed through analog multiplexer MUX type 506 andappears on the cathode on diode CR1. If any leakage occurs in theISOLATED BURN SENSOR LAYER then such leakage, necessarily to a voltageor ground which is more positive than -7.5 volts d.c. within the printedcircuit assmebly, will cause pin 2 buffer amplifier AMP1 to go morenegative than pin 3. Such a condition will shortly be seen to beimpossible for the sensing of temperature, being that such would imply atemperature less than 0° C. which is not encompassable within thecalibrated range of the present system (for temperature sensing ofprinted circuit assemblies immersed in a fluid with a freezing pointequal to water, or, in other words, no temperature lower than that ofice is desired to be, or can be, sensed by the present circuit ascalibrated in the preferred embodiment). Responsively to the voltagedifferential between pin 2 and pin 3 of buffer amplifier AMP1 upon theoccurrence of a current leakage to the ISOLATED BURN SENSOR LAYER, avoltage will be developed at buffer amplifier AMP1 output pin 6 which,as transmitted through current limiting resistor R13 type fixed film0.25 W 2% 100K, will be received at pin 3 of the offset differentialamplifier AMP2, an integrated circuit operational amplifier type 741.Meanwhile, the -7.5 volt d.c. voltage arising from the resistancedivider composed of R10 and R11 is applied through resistor R12 typefixed film 0.25 W 2% 100K to pin 2 of the offset differential amplifierAMP2. The voltage difference is amplified and applied through resistorR16 type fixed film 0.25 W 2% 15 ohms to analog to digital converterCON. This voltage is digitalized in analog to digital converter CON,integrated circuit type AD570, during the true occurrence of aconversion time pulse received from timing T1 and emplaced on paralleldigital signal lines for receipt, through plug jacks J1 through JN, bymicroprocessor MP. If the microprocessor MP had directed and selected asensing of a dielectric failure sensor, then any non-zero digitalizedvoltage level received on these parallel signal lines represents adielectric breakdown and potential burn of the selected printed circuitassembly.

When the scan circuit is employed in the temperature sensing mode, latchL1 will be set, producing a logical High voltage output signal andresulting the ultimate saturated conduction of transistors Q1 and Q2.When transistor Q1 is conducting in saturation, the voltage drop acrossthis transistor has no effect upon the accuracy of the circuit, thevoltage drop being seen across pins 2 and 3 of buffer amplifier AMP1being solely a function of the voltage drop across zener diode CR5 andresistors R3, R4, and R5. This voltage drop will be nominally +6.20volts d.c. The voltage drop across zener diode CR5 and resistances R3,R4 and R5 gives the constant offset voltage which is required to convertdegrees Kelvin to degrees Centigrade, or in other words, a +273 degreesKelvin offset. When such an offset is applied, then 0 volts d.c.appearing between pins 2 and 3 of buffer amplifier AMP1 will correspondto 0° C. as sensed by temperature sensors/transducer S1. Fixed resistorR3, fixed film type 0.25 W 2% 18K ohms, and fine adjustment variableresistance R4, type 0.5 watts 5% 500 ohms wirewound, and course variableresistance R5, type 0.5 watts 5% 10K ohms wirewound, are adjusted duringcalibration so that this desired +275 degree Kelvin offset maybeobtained. The diode CR4, which conducted when the voltage on pin 6 ofbuffer amplifier AMP1 was negative relative to the voltage on pin 2during the monitoring of dielectric failure, now isolates resistor R2during the sensing of temperature. Instead, resistor R1, a precisionfixed film resistor type 0.25 W 1% 50K, converts the current oftemperature sensor/transducer S1 to voltage across pins 2 and 3 ofbuffer amplifier AMP1. This resistor R1 converts the current oftemperature sensor/transducer S1 to voltage at 50 millivolts per 1° C.,and is critical for accuracy. Capacitor C1, fixed tantalum dielectrictype 50 V 20% 0.01 microfarads is for noise suppression. The voltagedeveloped at pin 6 of buffer amplifier AMP1, equal in value to 50millivolts per degree Centigrade above 0° C. as sensed by temperaturesensors/transducers S1, is passed through resistor R13 to pin 5 of theoffset differential amplifier AMP2. The voltage reference arising fromthe voltage divider comprised of resistors R10 and R11 is similarlypassed through resistance R12 to pin 2 of the offset differentialamplifier AMP2. Parallel capacitor C2 type fixed ceramic dielectric 50 V20% 0.01 microfarads and resistor R14 type fixed film 0.25 W 2% 100Kserve as a filter. A feedback loop from pin 6 to pin 2 of offsetdifferential amplifier AMP2 is established through parallel capacitorC3, type fixed ceramic dielectric 50 V 20% 0.01 microfarads and resistorR15, type fixed film 0.25 W 2% 100K. The capacitor C4, type fixedceramic dielectric 35 V 20% 0.01 microfarads is for noise suppression.The voltage responsive to temperature sensing, offset to give 0 voltd.c. equal 0° C., and amplified at 50 millivolts per degree Centigradeabove this base, is passed through resistor R16 and digitalized inanalog to digital converter CON for issuance to microprocessor MP. Thetiming, as conducted in timing T1, of the receipt of all commands frommicroprocessor MP, and the subsequent issuance, across bidirectionaldata bus lines, of the digitalized results of the directed sensing isprimarily a function of the timed protocol of the digital interface. Inother words, the sequence of control effectuated in timing T1 will be afunction of the digital interface chosen, the construction of which isroutine to a practitioner in the arts. A sole consideration invoked bythe present scan circuit is that address selection as accomplished inlatches L2 and decoded in decoder D1, as well as the selection of thedielectric failure or the temperature sensor as is accomplished in latchL1, would normally be stabilized 50 milliseconds before the timing T1would stop the conversion pulse to the analog to digital converter CONfor digitalization of the selected (analog) signal. At this time thefinal digital conversion value is held in analog to digital converterCON. This digitalized value is accessible by microprocessor MP as theoutput of analog to digital converter CON, integrated circuit typeAD570J, at any subsequent time to this final conversion.

A suggested systems utilization of the sensors for detection of thecatastrophic failure of the dielectric, the improper connection and/orthe over temperature of a printed circuit card assembly via one wire bythe scan circuit of the present invention is as follows. Themicroprocessor control would, normally, send in cyclic rotation theaddresses, and receive the data, for interrogation of the temperaturesensor/transducers S1 upon a multiplicity of printed circuit assemblies.If a no response, indicating 0 volts corresponding to 0° C. or 5 voltsrepresenting 100° C., was received for some assembly or assemblies, itwould be particularly useful if a second temperature sensor/transducerS1 were available for interrogation upon the same physical printedcircuit assembly. If neither of two temperature sensor/transducers S1,as well as the isolated burn sensor layer, can be sensed as givinganything but a wrong response upon a single printed circuit assembly,then the logical conclusion is that such a printed circuit assembly isnot correctly pluggably connected. As well as the rotational, scanned,interrogation of the multiplicity of temperature sensor/transducers S1as reside on a plurality of printed circuit assemblies, some temperaturesensor/transducers S1 which are located within the coolant stream(either liquid or air) may also be interrogated. The microprocessor MPwould normally compare the results from temperature interrogationsoccurring at all locations with a priorly developed system temperatureprofile table to the end of alterting the system operator, and possiblyautomatically removing power, in the event of various levels of overtemperature conditions within various system locations. As a validationof the correct function of the scan circuitry, four addresses providingfixed current into analog multiplexor MUX may be sensed during eachcycle of checking the multiplicity of sensors upon the plurality ofprinted circuit assemblies. Each of these four addresses would produce acurrent indicative of a simulated fault: a short between ground and theisolated burn sensor layer (or ground), an open connection to the scancircuit, a first high temperature and a second over temperature. Themicroprocessor MP might simply observe these correct responses tosimulated fault conditions, or might even develop appropriate controlsignals which could be analyzed at a further systems level, such aspower control, in validation of even the correct operation of themicroprocessor MP in the systems detection and response to dielectricfailure and temperature faults. A microprocessor MP will normally rotatethe dielectric sensor and temperature sensor checks as performed upon amultiplicity of printed circuit assemblies in a cyclical continuousmanner for the entire time during which power is applied. If themicroprocessor MP ever fails to receive any sensor data from the scancircuitry whatsoever, then such scan circuitry may be deemed to havefailed and printed circuit assemblies scanned by this circuitry plus thescan circuit would be normally powered off (power control not shown) forrepair. It would normally be monitored and detected by microprocessor MPthat the scan circuit had failed to respond in time, nominally 100millieseconds. If any individual printed circuit assembly under test isever detected to exhibit a dielectric failure or an over temperaturecondition, the microprocessor MP would normally cause system powercontrol (not shown) to turn off such printed circuit assembly to preventfurther potential damage. It should be noted that such a microprocessorMP based scan of system status might be adversely affected if the systemwere partially carded; that is, certain printed circuit assemblies werewithdrawn. A simple solution would be to employ dummy cards having fixedresistors on them in place of the temperature sensor/transducer S1. Theintent of the scenario just described is to show that the method and theapparatus of the present invention are readily adaptable to timely,continuous, flexible, reliable, and visible-of-failure observations ofthe dielectric integrity and temperature status of a large multiplicityof printed circuit assemblies. By the nature of the temperature readoutmethod employed, a short or an open is not an acceptable level andtherefore the protection accorded by the present invention cannot bedefeated easily.

While a specific preferred embodiment of the invention has beendescribed in detail as two sensors, a time-multiplexing voltage biasingand sensing circuit allowing the joint interrogation of both via asingle wire, and a selection circuit allowing the rotationalinterrogation of a multiplicity of such sensor pairs, it will beunderstood that the basic principles of the invention may be utilized inalternative manner. Furthermore, although the invention has beenparticularly shown and described with reference to the preferredembodiments thereof, it will be understood by those skilled in the artthat various alterations in form and detail may be made therein withoutdeparting from the spirit and scope of the invention. For example, themicroprocessor interface and the resultant capability to address amongsta multiplicity of sensor pairs may be eliminated and the present circuitis reducible to simply the time-multiplexed analog interrogation of asensor pair. For example, the dielectric failure sensors may beeliminated and a multiplicity of temperature sensor/transducers may beselectively addressably interrogated when such temperaturesensor/transducers are not located upon a multilayer printed circuitassembly. For example, the temperature sensor/transducers may beeliminated and a multiplicity of dielectric failure sensors located upona plurality of multilayer printed circuit assemblies may be selectivelyaddressably interrogated. For example, each sensor could be interfacedvia a separate wire separately biased to the analog multiplexor MUX asan individual address, and subsequently converted from current tovoltage, amplified, and encoded (although such division would sacrificethe desirable economy of utilizing a single wire to two sensors). Forexample, the circuit of the present invention could be employed withsubstitutionary, alternative, sensors giving a current outputresponsively to conditions such as pressure. For example, the circuit ofthe present invention could be employed with a temperature sensor in analternative range and with an alternative calibration.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A method of detectingthe breakdown by burning of the electrically insulating dielectricwithin a multilayer printed circuit board, said methodcomprising:establishing an electrically conductive layer within saidmultilayer printed circuit board which is normally continuallyelectrically isolated by electrically insulating dielectric from anyother voltage, signal, or ground as is transmitted upon other layers ofsaid multilayer printed circuit board during operational usage of saidmultilayer printed circuit board; electrically biasing duringoperational usage of said multilayer printed circuit board saidelectrically isolated conductive layer with a first voltage of greatervoltage magnitude than any other voltage, signal, or ground as istransmitted upon other layers of said multilayer printed circit boardduring said operational usage of said multilayer printed circuit board;sensing during operational usage of said multilayer printed circuitboard said first voltage in order to determined IF a first level of saidfirst voltage is sensed THEN substantially no current leakage betweensaid electrically isolated layer and any other said voltage, signal, orground within said multilayer printed circuit board is transpiring ELSEIF a second level of said first voltage is sensed THEN conductivecurrent flow is transpiring between said electrically isolated layer andat least some one of said other voltage, signal, or ground within saidmultilayer printed circuit board; wherein said sensing determination ofsaid second level of said first voltage means that said dielectricelectrically insulating said isolated conductive layer from at leastsome one of said other voltage, signal, or ground within said multilayerpritned circuit board has failed by becoming electrically conductivewhich occurs through carbonization attendant upon burning during saidoperational usage of said multilayer printed circuit board.
 2. Thedielectric breakdown defection method of claim 1 wherein saidelectrically biasing further comprises:electrically biasing through adiode said electrically isolated conductive layer with a first voltagewhich is both of correct polarity to forward bias said diode and whichis of greater voltage magnitude than any other voltage, signal, orground as is transmitted upon other layers of said multilayer printedcircuit board.
 3. The dielectric breakdown detection method of claim 2wherein said electrically biasing further comprises:electrically biasingupon a first time period through a diode said electrically isolatedconductive layer with a first voltage which is both of correct polarityto forward bias said diode and which is of greater voltage magnitudethan any other voltage, signal, or ground as is transmitted upon otherlayers of said multilayer printed circuit board, and electricallybiasing upon a second time period said diode with a second voltage whichreverse biases said diode; thereby electrically isolating through saiddiode said conductive layer during said second time period.
 4. A circuitapparatus for detecting the breakdown into conduction of the normallyelectrically insulating dielectric of a multilayer printed circuitboard, said apparatus comprising:a electrically conductive layer withinsaid multilayer printed circuit board normally continually electricallyisolated by electrically insulating dielectric from any other voltagesignal, or ground as is transmitted upon other layers of said multilayerprinted circuit board during operational usage of said multilayerprinted circuit board; voltage biasing means connected to saidconductive layer for charging during operational usage of saidmultilayer printed circuit board said normally electrically isolatedconductive layer to a non-zero voltage; and sensing means connected tosaid conductive layer for determining during operational usage of saidmultilayer printed circuit board if said non-zero voltage biased saidnormally electrically isolated conductive layer does connect to any oneor ones of other voltages, signals, and ground as occur on otherconductive layers of said multilayer pritned circuit card; wherein ifsaid normally isolated conductive layer is sensed during operationalusage of said multilayer printed circuit board to be connecting to othervoltages, signals, and/or grounds then said insulating dielectric ofsaid printed circuit card has broken down into conduction.
 5. Thedielectric breakdown sensing apparatus of claim 4 which furthercomprises:digitalizing means for converting said determining of saidsensing means into digital data for transmission upon a digitalinterface.
 6. A method for sensing upon a first time the failure of thedielectric and upon a second time the temperature of a multilayerprinted circuit card, said method comprising:electrically connecting afirst end of a first diode to a conductive burn sensor layer, said burnsensor layer normally continually electrically isolated from all otherconductive layers, including the layer carrying signal ground, withinsaid multilayer printed circuit card during operational usage of saidmultilayer printed circuit board; electrically connecting a second endof a second diode to a temperature sensor/transducer generating currentresponsively to temperature located upon said printed circuit card,which sensor/transducer is also electrically connected to signal ground;electrically wire connecting said second end of said first diode andsaid first end of said second diode and a voltage biasing and sensingcircuit, with said signal ground as a signal return path; then upon afirst time first voltage biasing with said voltage biasing and sensingcircuit upon said first time said second end of said first diode in apolarity wherein said first diode conducts current, while said firstvoltage biasing of said first end of said second diode causes saidsecond diode not to conduct current; while first electrically sensingwith said voltage biasing and sensing circuit upon said first time ifcurrent responsive to said first voltage biasing is conducted throughsaid current conducting first diode and through said conductive burnsensor layer to any others of said conductive layers within saidmultilayer printed circuit card from which said conductive burn sensorlayer is normally electrically isolated; then upon a second time secondvoltage biasing with said voltage biasing and sensing circuit upon saidsecond time said first end of said second diode in a polarity whereinsaid second diode conducts current, while said second voltage biasing ofsaid second end of said first diode causes said first diode not toconduct current; while second electrically sensing with said voltagebiasing and sensing circuit upon said second time such conduction ofcurrent responsive to said second voltage biasing as occurs through saidcurrent conducting second diode and said temperature sensor/transducer;wherein said first time first electrically sensing of any currentconduction is an indication of the failure of the dielectric insulationupon said multilayer printed circuit card; wherein said second timesecond electrically sensing of current conduction is an indication ofthe temperature of said temperature sensor/transducer upon said printedcircuit card.
 7. The dielectric failure and temperature sensing methodof claim 6 which further comprises:first performing said first voltagebiasing and said first electrically sensing responsively to a firstdirective received upon a digital interface to said voltage biasing andsensing circuit, and second performing said second voltage biasing andsaid second electrically sensing responsively to a second directivereceived upon said digital interface; and first digitalizing in ananalog to digital converter results of said first electrically sensingfor transmission upon said digital interface responsively to said firstdirective, and second digitalizing in said analog to digital converterresults of said second electrically sensing for transmission upon saiddigital interface responsively to said second directive.
 8. Thedielectric failure and temperature sensing method of claim 6 whichfurther comprises:first performing and first digitalizing, and secondperforming and second digitalizing, as directed upon said digitalinterface for a selected and addressed one of a multiplicity of saiddiode connected burn sensor layers connected to a multiplicity of saidtemperature sensors/transducers as are located upon a plurality of saidmultilayer printed circuit cards, said selected and addressed one ofsaid multiplicity being selected and addressed in a multiplexorresponsively to said direction upon said digital interface.